Harmonic and phase shift suppressor means



March 2, 1965 E. B. HILKER HARMONIC AND PHASE SHIFT SUPPRESSOR MEANS Filed March 6. 1961 2 Sheets-Sheet 1 hill EH77? fmmv 51 Ila x5e le'aznssn,

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March 2, 1965 E. B. HILKER 3,172,034

HARMONIC AND PHASE SHIFT SUPPRESSOR MEANS Filed March 6. 1961 2 Sheets-Sheet 2 I: Wrfar United States Patent 3,172,034 HARMQNIKI AND PHASE SHIFT SUPPRESSGR MEANS Erwin B. Hillier, deceased, late of St. Louis, Me, by Annamary Hillier, administratrix, St. Louis, M0, assignor to Wagner Electric Corporation, St. Louis, Md, a corporation of Delaware Filed Mar. 6, 1961, Ser. No. 93,807 13 Claims. (Cl. 323-75) The present invention relates generally to electric control devices such as voltage control devices and the like and more particularly to means employed in conjunction with such devices for reducing or eliminating undesirable harmonics and phase shift in the output thereof. Devices of interest in connection with the present device ars shown in my Patents Nos. 2,931,968, 2,931,969 and 2,931,970, all of which issued April 5, 1960. Other patents owned by the inventors assignee and which are also of interest are J. S. Malsbary Patent No. 2,871,373

issued January 27, 1959; J. S. Malsbary Patent No. 2,892,146 issued June 23, 1959 and J. S. Malsbary Patent No. 2,960,646 issued November 15, 1960.

A voltage control device of the type disclosed in the aforementioned patents includes means for providing a variable compensating or adjusting voltage which is superposed on, or injected into, either the input or output voltage of a transformer or other alternating current supply device so as to maintain the output voltage at substantially the desired value regardless of changes in the supply voltage within predetermined limits. The compensating voltage is developed in a bridge circuit, as for example, a Wheatstone type bridge circuit of four saturable core reactors whose impedances are made responsive to selected external conditions such as voltage or current; the magnitude and phase of the compensating voltage being determined by the relative impedance values of the arms of the bridge.

Although the aforementioned control devices have operated very satisfactorily, it has been found that under certain operating conditions the output voltage contains so-called higher harmonics which may interfere with telephonic communications; also, there is at times an undesirable phase shift between the supply and output voltages.

The harmonic and phase shift suppression means disclosed in Patent No. 2,931,970 substantially eliminates theharmonic components otherwise present in the output voltage, and maintains the phase shift between supply voltage and output voltage small and substantially constant. In the patented devicethe suppressor means comprises a variable impedance device (shown in the preferred embodiment as a saturable core reactor) connected across the bridge circuit in which the compensating volage is developed; with means for manually controlling the impedance of the reactor or other variable impedance device.

In Patent No. 2,931,968 automatic means are provided in conjunction with the means for suppressing harmonics and phase shift. The automatic means comprise a variable impedance device connected across the bridge circuit in which the variable voltage is developed, and means for automatically varying the impedance of the variable impedance device in response to an electrical condition to be controlled whereby the impedance of the device is relatively low when the electrical condition is at a predetermined desired value, and relatively high when the electrical condition varies from the desired value.

Patent No. 2,931,969 discloses still other means for varying the impedance of a device connected across the output terminals of a bridge circuit.

The present invention also concerns a phase shift and harmonic suppressor and includes means for automatically controlling the suppressor in response to preselected conditions. Briefly, the present invention comprises control winding means mounted on or in association with selected ones of the impedance elements of the bridge circuit in which a variable control voltage is developed, and means for automatically energizing the said control windings in response to variations in an electrical condition to be controlled whereby said control windings vary the impedance of the associated bridge elements so that the impedances are relatively low when the electrical condition is at a predetermined desired value, and relatively high when the electrical condition varies from the desired value. In the present device the impedance elements associated with the control windings are in adjacent legs of the brdige.

It is an object of the present invention to provide novel control means for automatically controlling a harmonic and phase shift suppressor of the aforementioned type. More particularly, it is an object to provide such control means which are responsive to the variations in selected electrical conditions in the circuit, as for example, voltage and current values.

Another object is to provide an electric control system of the aforementioned type wherein the output voltage of the system is automatically maintained substantially free of harmonic components.

Another object is to provide an electric control system of the aforementioned type wherein the phase shift be-' tween the supply voltage and output voltage of the system is automatically maintained small and substantially constant.

Another object is to provide a novel fully automatic voltage control system having a controlled saturable core reactor bridge providing a compensating voltage for maintaining an electrical condition of the system substantially constant, wherein the phase shift between the supply voltage and output voltage of the system is maintained small and the harmonic components otherwise present in the' output voltage are greatly reduced.

Another object is to provide means for suppressing harmonics and phase shift in a control system without requiring any additional circuits in series with the main flow of energy.

Another object is to provide auxiliary means for modifying the impedance of the elements of a bridge control circuit in a manner to reduce harmonics and phase shift in the output of a system controlled thereby.

Another object is to reduce the number of components of a bridge circuit employed to regulate voltage in a supply system, which bridge circuit also includes means for suppressing harmonics and phase shift.

Further objects and advantages of the present invention will be apparent from the detailed description, reference being had to the accompanying drawings wherein preferred embodiments of the present invention are shown.

In the drawings:

FIG. 1 is a schematic diagram of a preferred embodiment of the present invention; 1

FIGS. 2(a), (b), and (c) are schematic wiring diagrams illustrating three possible operating conditions of the bridge circuit per se of FIG. 1;

FIG. 3 is a graph of a typical characteristic curve of the voltage detector 52 of FIG. 1;

FIG. 4 is a chart containing typical control characteristic curves of each of the amplifiers 60 and 62 of FIG. 1, and another curve representing a resultant of these curves;

FIG. 5 is a chart containing a typical control characteristic curve of the amplifier 96 of FIG. 1;

FIG. 6 is a graph of the output current of the ampliher 96 over a typical control range of the system; and

FIG. '7 is a chart similar to the chart of FIG. 4 but illustrating the elfect on the impedance of the bridge reactors produced by the control windings 41 and 43.

, Referring to the drawings and particularly to FIG. 1, there is shown'for illustration an automatic voltage control device which is of the type shown and described in Patent No. 2,892,146. Shown in combination with the control device is a harmonic and phase shift suppres sor circuit, and suppressor control means 14 for automatically controlling the impedance of the suppressor circuit.

The automatic voltage control circuit 10 includes a power transformer 16, a. compensating voltage device 18, and control means for automatically controlling the operation of the device 18 to maintain the output voltage V of the transformer 16 substantially constant regardless of changes in supply voltage V within predetermined limits. It will be assumed for purposes of illustration only, that the output voltage V is held substantially constant .while the supply voltage V may vary within the range of i 10% of the normal or desired value.

The transformer 16 includes a primary winding 22 which is connected through the compensating voltage device 18 and power lines 23'and 24 to an A.C. voltage sup-- ply sourceindicated at 25, a secondary winding 26 which is connected to conductors 27 and 28 for supplying power to a load 29,.and a corrector winding 30 which will be considered hereinafter.

The compensating voltage device 18 includes a Wheatstone type bridge circuit 32 shown as having four saturable core reactors 34, 36, 38 and connected together to form the four impedance arms of the bridge. Each reactor has a saturable magnetic core on which is positioned an alternating current reactance winding, and a direct current control winding for controlling the magnetic saturation'of the core and the impedance of the A.C. winding; The windings of each reactor are identified in the drawing by corresponding like numerals with the letters AC or DC added thereto.

. The windings 34AC and 36AC are connected together at a corner 42, the windings 36AC and 38AC are connected togetherat a corner 44, the windings 38AC and 40AC are'connected together at a corner 46, and the windings 40AC and 34AC are connected together at a corner 48.

The control windings 34DC and 38DC of the bridge circuit 32 are connected in series circuit relation, and a-signal applied to this pair'of series connected control windings will determine the impedance of the diametrically opposite reactance windings 34AC and 38AC. The control windings 36DC and- 40DC are also connected in series circuit relation, and'a signal applied to this pair of control windings will determine the impedance of the other diametrically opposite reactance windings 36AC and 40AC. e

. Although the bridge circuit 32 is shown including four separate cores with an A.C. reactance winding and a DC control winding on each-core, other winding and corearrangements may be used if desired. For example, each set of diametrically opposite reactors may include'twoseparate coreseach having an A.C. reactance winding thereon, and both of said cores can be coupled by'a common DC. control winding. If this construction were used then one core in each set of reactors would also include another control winding as will be described hereinafter.

A conductor 49 connects one end of the primary winding 22to the corner 48 of the bridge circuit, and the diametrically opposite corner 44 of the bridge circuit is connected to the power line 23. The other diametrically opposite corners 42 and 46 are connected across the corrector winding 30 through the conductors 50 and 51, respectively.

The corrector winding 30 provides a voltage which is impressed across the corners 42 and 46 of the bridge circuit so as to provide a compensating or adjusting voltage e which appears across the other corners 44 and 48 of the bridge. It will be assumed for purpose of discussion, that the number of turns of the corrector winding 30 is 10% of the number of turns of the primary winding 22 so that the voltage across the corrector winding is 10% of the induced voltage, thus permitting compensation for approximately a 10% change in the supply voltage. The compensating voltage e is effectively in series with the primary winding voltage E, and, depending upon the balance conditions or relative impedances of the two pairs of diametrically opposite reactors of the bridge circuit, the voltage e is either substantially ineffectual, aiding, or opposing the supply voltage V to thereby affect the voltage E in such manner as to maintain the magnitude of the output voltage V substantialw ly constant at the desired or normal value.

In the illustrated embodiment of FIG. 1, the control circuit 20 automatically controls the balance conditions of the saturable core reactor bridge 32 by supplying control signals (responsive to the small variations in the con trolled output voltage V to the DC. control windings of the reactors of the bridge circuit. The control circuit 20 includes a voltage detector 52 having a pair of input terminals 54 shown connected across the output circuit of the transformer 16 through conductors S5 and 56, and a pair of detector output terminals 58 connected to supply control signals to a pair of amplifiers 60 and 62. The output of these amplifiers supply the current to the DC. control windings of the bridge.

The amplifiers 60 and 62 shown in the drawing are self-saturating magnetic amplifiers of the well known.

type. The amplifier 60 includes two saturable magnetic cores 63 and 64 having thereon reactance or power windings 65 and 66 respectively, DC. control windings 67 and 68 respectively, and DC. bias windings 69 and 70, respectively. The power windings 65 and 66 are each connected in a branch circuit with the branches being connected in parallel between common junctions 71 and 72 of the two branches. The junction 71 is connected to an A.C. input terminal 73 of a full-wave bridge-type rectifier 74 while the junction 72 is connected to a terminal 75 which in turn connects with the conductor 55. Another A.C. input terminal 76 of the rectifier 74 is connected to a terminal 78 which in turn connects with the conductor 56. The terminals 75 and 78 form the A.C. power input terminals of the amplifier while the terminals 73 and 76 form the A.C. power output terminals of the amplifier as well as the A.C. input terminals of the rectifier 74.

A one-way valve or half-wave rectifier 80 is connected in series with the power winding 65 and a half-wave rectifier 81 is connected in series with the power winding 66. The half-wave rectifiers 80 and 81 are oppositely related or poled with respect to a supply voltage applied to the amplifier power input terminals 75 and 78 so that the rectifiers conduct current on opposite half cycles of the power supply voltage, and provide an A.C. output at the amplifier output terminals 73 and 76. Thus, halfwave or intermittent unidirectional current flows in each of the power windings 65 and 66 generating unidirectional M.M.F.s tending to saturate the cores and reduce the impedance of the power windings. The magnetic saturation resulting from these M.M.F.s is referred to as self-saturation and the direction or sense of these M.M.F.s as indicated by the arrows adjacent the power windings, is referred to as the saturating direction. M.M.F.s in the opposite sense tend to increase the impedance of the power windings and are known as desaturating M.M.F.s and the direction or sense of these M.M.F.s is referred to as the desaturation direction.

The relative directions of the M.M.F.s resulting from current flowing in the various bias and control windings are indicated by arrows adjacent these windings.

In general, in an arrangement employing two separate cores per amplifier, each core carries one power winding and at least one control winding, while if the cores are close together or if the well known single three-legged core is employed, a single control winding may encircle both cores or the center leg of the three-legged core. In any case, the induced alternating current voltages or the alternating fluxes due to the pulsating current through each power winding are made to cancel out with respect to the control windings and bias windings if any are used.

The self-saturating amplifier 62 is similar to the amplifier 60 and like parts are identified by like numbers except that the numbers are primed. The AC. power input terminals of both amplifiers are shown connected to the same power source, i.e., both are connected to the conductors 55 and 56 which in turn are connected across the output conductors 27 and 23 of the transformer 16. The AC. output current of the amplifier 60 is rectified by the full-wave rectifier 74 with D.C. output terminals 83 and 84 of the rectifier being connected to supply D.C. control currents to the reactor bridge control windings 36DC and 4013C through an adjustable resistance 85. The A.C. output current of amplifier 62 is rectified by the full-wave rectifier 74 with D.C. output terminals 83 and 84 of this rectifier connected to supply DC. control currents to the reactor bridge control windings 3413C and 38DC through an adjustable resistance The control windings 67, 68, 67 and 68' are connected in series and are'connected to the DC. output terminals of a full-wave rectifier 87 through an adjustable resistance 88. The AC. input terminals of the rectifier 87 are connected to the output terminals 58 of the voltage detector 52.

The control windings 67 and 68 are wound and connected so that current flow through these windings results in desaturating M.M.F.s which tend to decrease the output of amplifier 60. The control windings 67 and 63', however, are wound and connected so that current flowing through them results in saturating M.M.F.s which tend to increase the output of amplifier 62. Thus, the control windings are energized in response to the output of the voltage detector, and, as indicated by the arrows adjacent the control windings of each amplifier, the current supplied by the detector 52 causes the output current of one of the amplifiers to increase while the output of the other tends to decrease for a given change in current supplied by the detector.

The bias windings of each of the amplifiers are connected to any suitable current source. As illustrated in the drawing, the bias windings 69 and '70 of amplifier 66 are connected to a battery 90 through an adjustable resistance 91. The direction of bias current is such that the M.M.F. produced thereby is in the saturating direction as indicated by the arrows adjacent the windings 69 and 70. The magnitude of the bias current is so adjusted that when a small or zero signal is applied to the control windings 67 and 68 the output of the amplifier 60"is high or at its maximum value. The bias windings 69 and 70 of amplifier '62 are shown connected to be supplied from a battery 94) through an adjustable resistance 91', andthe direction of bias current in these windings is such that the produced is in the desaturating direction as indicated by the arrows adjacent these windings. The magnitude of this bias current is so adjusted that when a small or Zero signal is applied to the control windings 67 and 68' the output of amplifier 62 is low or at its minimum value.

The voltage detector 52 acts as a current regulating Valve which does not permit an appreciable current to flow therethrough until the voltage applied to its input terminals 54'exceeds a predetermined critical value, and then the increase in current flow to its output circuit is in direct proportion to the increase in voltage above that critical value.

It will be assumed herein for illustration only that the output voltage V is to. be maintained within i'l% of the predetermined desired or normal value even though the supply voltage may vary from to 110% of its normal value. Thus, in FIG. 3, there is shown a typical characteristic curve of the detector 52 in which the output current of the detector (going to the control coils 67, 68, 67' and 68') is along the abscissa axis and the output voltage V in percent of the normal value, is along the ordinate axis. There is also shown in parenthesis along the abscissa axis the corresponding supply voltage in percent of its normal value. It is seen from this curve that the detector has a low or negligible output current when the output voltage V is at its minimum permissible value (99% of the normal value) and that the output current increases rapidly and substantially in proportion with an increase in output voltage V above the minimum permissible value.

Any suitable Voltage detector having a similar characteristic curve as that shown in FIG. 3 may be used. The detector 52, shown in FIG. 1, is of a well known type which consists simply of a coil wound on a saturable core and designed such that the core tends to saturate at a desired magnitude of voltage applied to it so that the current through the coil increases rapidly after saturation.

Referring to FIG. 4, the curve A represents the control characteristic of amplifier 60 (i.e., the output of amplifier 60 to coils 36DC and 4013C, based on the current flowing from the detector 52) and the curve B represents the characteristic curve or" amplifier 62 (i.e., the output of amplifier 62 to coils 34DC and 38DC, based on the current flowing from the detector 52). It will be noted that for a negligible or zero control current applied to the control windings 67, 68, 63' and 67' by the detector 52, the output of ampl fier 66 is at a high or maximum value, such as indicated by a point a on curve A, while the output of amplifier 62 is at a minimum value, such as indicated by a point b on curve B. As the output current of the detector increases from the negligible value, the output of amplifier 60 (curve A) decreases from the maximum value (point a to a minimum value such as that at point a while the output current of amplifier 62 (curve B) increases from the minimum value (point h to a maximum value such as that at point b Thus, it is seen from FIGS. 3 and 4 that the output of amplifier 62 is directly proportional to the magnitude of the output voltage above the minimum permissible value, while the output of amplifier 6G is inversely proportional to the magnitude of output voltage above the minimum permissible value.

The two characteristic curves A and B are shown on the same graph (FIG. 4) and cross at a point p where the output current of each of the amplifiers is the same and at a relatively low value. The magnitude of the output of each amplifier being the same at point p, the two pairs of diametrically opposite reactors of the bridge 32 are equally energized and the bridge 32 is in balance. It will be apparent that the shape of the curves A and B and the location of the crossover point p can be varied, as for example, by changing'the ratio of the current in the bias windings 6% and 70 with respect to the current in the bias windings 69' and '70.

The operation of the complete voltage control device It is described in detail in the previously mentioned Patent No. 2,892,146 and will only briefly be described here When the output voltage V is at its normal value, no adjusting or compensating voltage e is required. Therefore, when the output voltage V is at 100% of its normal value (FIG. 3) the amplifiers 60 and 62 provide equal control currents (point p, FIG. 4) to the two pairs of diametrically opposite bridge reactors so that the bridge 32 is balanced. Thus, assuming that the Voltage drops across the reactors in the bridge are equal and cophasal (a condition which will be discussed more fully hereinafter) the voltage across bridge corners 44 and 48 will be relatively low and substantially ineffectual with regard to the magnitude of the output voltage V If the supply voltage V increases above its normal value, the output voltage V tends to increase, causing an increase in the output current of the detector 52 which, in turn, increases the output current of amplifier :62 while tending to decrease the output current of amplifier 60, as seen by the curves A and B in FIG. 4. The increase in output current of amplifier 62 tends to saturate the bridge reactors 34 and 38 so that the impedance of windings 34AC and 38AC decreases, while the decrease in the output current of amplifier 61) tends to increase the impedance of reactor windings 36AC and AC. Thus, the bridge circuit 32 is unbalanced and an adjusting voltage e appears across the terminals 44 and 43 of the bridge. This adjusting voltage e is of such magnitude and relative polarity or phase angle with respect to V that it opposed or bucks V thereby reducing the primary voltage B so as to compensate for the increase in V and to maintain the output voltage V substantially constant i.e., within its predetermined limits.

On the other hand, if the supply voltage V decreases below the normal value, the output voltage V tends to decrease, causing a decrease in the output of detector 52 which in turn causes an increase in the output of amplifier while causing a decrease in the output of amplifier 62. The increase of output current of amplifier 61) tends to saturate the bridge reactors 36 and 40 and to reduce the impedance of windings 36AC and 4tlAC, while the decrease in output current of amplifier 62 tends to increase the impedance of bridge reactor windings MAC and 38AC. The bridge circuit, under these conditions, is again unbalanced and an adjusting voltage e appears across terminals 44 and 48 of the bridge. In this case however, the adjusting voltage is of such magnitude and relative polarity or phase angle with respect to V that its adds to V thereby increasing the primary voltage E so as to compensate for the decrease in V and maintain the output voltage V substantially constant within its predetermined limits.

Up to this point it has been assumed that the voltage drops across the bridge reactors are cophasal in order, to simplify the theoretical explanation of the control circuit 10. The cophasal relationship between the voltage drops across the bridge reactors is very nearly obtained when the current flowing in the primary 22 of the transformer 16 is small in comparison with the current fiowing in the corrector winding 30. However, these cophasal voltage conditions exists only rarely. When the transformer is loaded, the voltage drops across the different reactors of the bridge circuit are not eophasal due to load current flowing through the bridge reactors, and the voltage relations become much more complex. For example, it has been found that under load conditions, when the bridge is balanced, i.e., when equal currents flow in the DC. control windings of the bridge reactors, an undesirable voltage e appears across the corners 44 and 48 which is approximately at right angles to the voltage across Corrector winding 30 and the voltage E across the primary winding 22. In such a case, the magnitude of the primary voltage E is close to its normal valve and the magnitude of voltage V is close to its normal value regardless of the fact that the voltage e appears across the corners 44 and 48 of the bridge. How ever, the primary voltage E, and therefore the voltage V is appreciably shifted in phase with respect to the supply voltage V It has been found that this undesirable voltage e contains relatively high harmonies which are transformed into the output voltage harmonics which are transformed into the output voltage V and which may interfere with telephonic communications if the voltage control device 111 is used to transmit power along lines which are close to telephone lines.

On the other hand, when the bridge circuit 32 is unbalanced, as for example when the ratio of the imped- 8: ance values of the reactance winding 40AC to the re actance winding MAC and the ratio of the impedance values of the reactor winding 36AC to reactor winding 38AC are rather small, the voltage e across the corners 44 and 48 attains a nearly cophasal relationship with respect to the voltage across the corrector winding 30 or corners 42 and 46, and also with respect to the primary voltage E. Consequently, in such case the phase displacement between V and V is small. Also, the magnitudes of the harmonics in the compensating voltage under" unbalanced bridge conditions are low. I

Thus, when the input voltage V and the output volt age V are normal, the bridge circuit is in balance anda voltage 2 exists across bridge corners 44 and 48 which is substantially at right angles to the primary or induced voltage E, and which results in relatively high harmonics in the output voltage V and an appreciable phase displacement between V and V Consequently, when the output voltage is normal and a compensating voltage is not needed but does exist, it is advantageous to reduce the magnitude of this compensating voltage e to aminimum, and this is a function of the present invention. In the illustrated embodiment of the present invention, the undesirable compensating voltage e which appears across terminals 44 and 48 of the bridge when the output voltage V is normal, is greatly reduced so as to reduce the harmonics in the output voltage to a minimum and also to greatly reduce the phase shift between the supply voitage V and output voltage V The suppressor control circuit 14 in FIG. 1 includes a self-saturating magnetic amplifier 96 which is somewhat similar to the amplifiers 60 and 62. The amplifier 96 is controlled in response to output voltage variations and its output is supplied to control windings 41 and 43. The control winding 41 is mounted on or in association with the bridge reactor 38 and operates in conjunction with the winding SSDC to control the impedance of the reactor winding SSAC. Similarly, the control winding 43 is mounted on or in association with the bridge reactor 40 and operates in conjunction with the winding 4DC to control the impedance of the winding 40AC. It is also contemplated to employ the control windings 41 and 43 in conjunction with other adjacent reactors of the bridge circuit 32 such as the reactors 34 and 36 without changing the nature of the invention.

The amplifier includes two saturable magnetic cores 98 and 99 containing reactance or power windings 100 and 102, respectively, control windings 104 and 105, respectively, control windings 106 and 107, respectively, and bias windings 108 and 109, respectively.

The power windings 1% and 102 are connected in a pair of parallel branch circuits 110 and 112, respectively, with the opposite ends of the branches having common unctions 114 and 115. A half-wave rectifier 116 is connected in branch 110 in series with the power winding 1%, and a half-wave rectifier 117 is connected in branch 112 in series with power winding 102. The junction 114 is connected to terminal 118 of a pair of amplifier A.C. output terminals 118 and 118, while the junction is connected to terminal 126 of a pair of amplifier A.C. power input terminals 120 and 120'. The power input terminal 120' and the power output terminal 118 are connected together.

The power input terminals 120 and 120 are connected to an A.C. power source indicated by the numeral 122, while the A.C. output terminals 118 and 118' of the amphfier are connected to a full wave rectifier 124 whose DC. output terminals are connected to opposite ends of the control windings 41 and 43 as shown in FIG. 1. The power supply source 122 for the amplifier may be any suitable A.C. source, for example, the terminals 129 and 120 may be connected across the substantially constant output voltage circuit of the transformer 16.

The half-wave rcctifiers 116 and 117 are oppositely poled or related with respect to the supply voltage-applied to the power input circuit 120120' so that the rectifiers conduct current on alternate half cycles of the supply voltage and provide an alternating current at output terminals 118 and 118. Thus, intermittent pulsating unindirectional current flows in the power windings 161i and 1192 and produces saturating M.M.F.s which tend to saturate the cores and increase the output of the amplifiers. The direction of these saturating M.M.F.s are indicated by the arrows adjacent the power windings 100 and 102.

The pair of control windings 104 and 105 of amplifier 96 are connected in series and connected by conductors 126 and 127 through an adjustable resistance 128 across the D.C. output terminals 83 and 84 of the rectifier 74 and are therefore energized in response to the output of amplifier 61). The pair of control windings 106 and 107 of the amplifier 96, in like manner, are connected in series and connected by conductors 129 and 130 through an adjustable resistance 131 across the DC. output terminals 83 and 34 of the rectifier 74' and therefore are energized in response to the output of amplifier 62. Thus, in the circuit of FIG. 1, the outputs of amplifiers 6t) and 62 control the relative impedances of the reactors in the bridge 32 and also determine the output of amplifier 96, which in turn controls the current in the control windings 41 and 43.

The bias windings 163 and 169 are shown connected in series and'to a bias current source indicated as a battery 133 and a series adjustable resistance 134. As indicated by the arrows adjacent these bias windings, the M.M.F.s resulting from bias current flowing therethrough are in the saturating direction tendingto raise the output of the amplifier. The magnitude of bias current for amplifier 96 is adjusted so that for a minimum total control current flowing in the control windings 1G4, 106, 107 and 105, the output of the amplifier 96 is at its maximum value.

The control windings 104, 105, 1% and 197 are so connected and wound that the M.M.F.s resulting from current fiow in these windings are in the desaturating direction as indicated by the arrows adjacent the windings, that is, these M.M.F.s oppose saturation and tend to decrease the output of the amplifier 56. Thus, the output currents from both of the amplifiers 6G and 62 affect the amplifier 96 in the same sense so that their combined effect may be represented by the curve C in FIG. 4. With the output current of each of the amplifiers 60 and 62 at a relatively low value at the crossover point p, the change in output current of one of the amplifiers will be greater than the change in output current of the other for a given change in the output current of the detector. Curve C represents the control ampere-turns of amplifier 96 and is obtained by adding the instantaneous values of the curves A and B.

FIG. 5 shows the control characteristic curve D of the amplifier 96 and illustrates the variation of the output current of the amplifier $6 as a function of the current into the control windings 105, 164, 1116 and 107. As already noted all ofthesecontrol windings are wound so as to produce a similar effect on the output as indicated by the directions of the arrows in FIG. 1. The graph D of FIG. 5 illustrates that increasing the current totheco-ntrol windings 164, 105, 166 and 107 produces a corresponding decrease in the output current from the amplifier 96, and in turn decreases the current in the control windings 41- and 43. This tends to increase the impedance of the reactors 38 and 40. FIG. 6 also illustrates the variation of the output of the amplifier 96, but in relation to changes of the supply voltage.

It should be noted in this connection that the control windings 41 is wound to produce a field which aids the field produced by the associated. reactor control winding 38130 as indicated by the arrows on FIG. 1, and the winding 43 similarly is wound to aid its associated reactor control winding 40DC. Therefore, when relatively large current is produced in the output of the amplifier 96 the impedances of the reactors 3S and 41? will de crease, and when the current from the amplifier 96 decreases the impedance of the reactors 38 and 41) will tend to increase.

Two different extreme conditions of bridge unbalance and one condition of bridge balance are illustrated in FIG. 2. In FIG. 2(a) the impedance of the reactors 34 and 38 are shown as short circuits and the impedances of the reactors 36 and 48 are shown at some higher values. This is the condition existing when the output current of amplifier 61 is relatively small and the output of the amplifier 62 is relatively high. FIG. 2(b) illustrates the opposite condition wherein the impedances of the reactors 36 and 41) are at or near short circuits.

FIG. 2(0) illustrates the condition when the bridge 32 is in balance, that is when the output of the amplifiers 6t and 62 are substantially equal. In this condition it is desired to reduce the impedances of the reactors 38 and 40 associated with the control windings 41 and 43 in order to effect reduced impedance or a short circuit across the output terminals 44 and 48. of the bridge. This is accomplished at balanced condition because then the output of the amplifier 96 is relatively high, and therefore produces considerable current flow in the windings 41 and 43 to thereby reduce the impedances of the associated reactors 38 and 4%.

FIG. 7 illustrates graphically how the impedances of the reactors 34, 36, 38 and 4t? vary with changes of the supply voltage above and below the normal supply voltage condition. In the graph an impedance curve of each reactor is shown and is identified by the same number as its corresponding reactor. For example, curve 34 represents the reactance (impedance) of the reactor 34 as a function of supply voltage; curve 36 represents the reactance (impedance) of the reactor 36; curve 33 represents the reactances (impedances) of the reactor 38 which is controlled by the combined action of the control windings 38DC and 41 and curve 40 represents the reactance (impedance) of the reactor 40 which is controlled by the combined action of the control windings NBC and 43. It should be noted in connection with curves 38 and 40 that the impedances of the associated reactors are substantially reduced at and around normal supply voltage which condition is brought about by the action of the control windings 41 and 43 respectively. It should also be noted that the effect on reactor impedance of the control windings 41 and 43 becomes relatively negligible as the supply voltage varies further and further from the normal value. This is precisely the condition desired because it enables :the bridge circuit to effectively control the output voltage as the supply voltage varies and therefore is relatively uneitected by the windings 41 and 43, while at the same time shorting out the bridge circuit to eliminate harmonics and phase shift when the bridge is at or near balanced condition which exists when the supply voltage is normal.

In operation, when the output voltage V is normal, the detector 52 provides a control signal (indicated at a point n in FIG. 3) to the control windings of the amplifiers and 62 which causes their respective outputs to be equal, as indicated at the cross-over point p in FIG. 4. This provides the two pairs of diametrically opposite control windings of the saturable reactor bridge circuit 32 with equal DC control currents whereby the bridge circuit is balanced. At the same time, the control windings 104, 105, 1136 and 107 of amplifier 96 are energized by a control signal of minimum value, as indicated at point In on curve C, so that amplifier 96 has a maximum output (indicated by point d on curve D, FIG. 5) to cause a relatively large current to flow through the control windings 41 and 43. Thus, when the output voltage V is normal, the bridge is balanced and the otherwise present undesirable voltage e, is made substantially ineffectual due to the fact that the bridge 32 issubstantially shorted.

On the other hand, if the output voltage V tends to vary above or below the normal value, the amplifier 96 automatically causes the current flow through the control windings 41 and 43 to decrease to thereby tend to increase the impedance of the reactors 33 and 49. In other words, the elfect of amplifier 96 on the impedance of reactors 38 and 40 thus diminishes as the bridge is unbalanced in either direction by amplifiers 60 and 62. This permits an increase in the compensating voltage e under conditions when the bridge 32 is unbalanced and enables the bridge to maintain the output voltage V within its predetermined limits.

Thus, for example, if the output voltage V tends to increase above its normal value due to the supply voltage V increasing to 110% of its normal value, the small increase in output voltage causes the output of the detector 52 to increase to a high value indicated at point 11 on the curve in FIG. 3, thereby increasing the output of amplifier (S2 to a relatively high value (point [7 curve B) of FIG. 4 while decreasing the output of amplifier 69 to a low value (point a curve A). This causes the saturable core reactor bridge 32 to become unbalanced and produce a correcting voltage e which opposes the impressed voltage V so as to maintain the primary voltage E substantially constant. At the same time, the resultant effective desaturating control M.M.F.s due to signal currents from the amplifiers tit) and 62, which are applied to the control windings of amplifier 96, are increased to a high value, as indicated by point C on curve C, and the output of amplifier 96 is decreased to a low value, as indicated at point d on curve D, FIG. 5. Since the output of amplifier 96 is at a low value, it has little or no effect on the impedance of reactors 38 and 40 under these conditions, thus the low impedance or short circuiting etfect which occurs under balanced bridge conditions is removed. Therefore, the voltage e is effective and opposes the supply voltage V thereby tending to reduce the primary voltage E and maintain the output voltage V within its predetermined limits.

In like manner, if the output voltage V tends to decrease from the normal value, for example due to the supply voltage decreasing to 90% of its normal value, the small decrease in output voltage causes the output current of detector 52 to decrease to a low value, such as indicated at point I on the curve in FIG. 3. The decrease in output current of the detector 52 causes the output of amplifier 6t) to increase (point a curve A) and the output of amplifier 62 to decrease (point b curve B) thereby producing control signals which cause the bridge circuit 32 to become unbalanced. At the same time, the desaturating control M.M.F.s due to signal currents from the amplifiers 6t? and 62, which are applied to the control windings of the amplifier as, are at a high value as indicated by a point C on curve C. Consequently, the output of the amplifier 96 is again reduced to a low value as indicated at point d on curve D, FIG. 5. This again dereases the effect of amplifier 96 on the impedances of the reactors 38 and 49 so that the bridge can perform its main function of regulating the output voltage V However, in this case the voltage e is aiding the supply voltage V tending to raise the primary voltage E to thereby maintain the output voltage V within its predetermined limits.

The voltage impressed across bridge corners 42 and 46 in FIG. 1 is obtained by means of the corrector winding 30 on the transformer 16, however, this voltage may be obtained by means of a separate transformer. For example, where it is desired to connect a power supply source directly to a load circuit without the use of a power transformer, such as transformer 16, the voltage impressed across these bridge corners may be obtained from a separate transformer connected across the load circuit.

While the bridge circuit 32 in FIG. 1 is shown connected in power line 23 in series with the primary winding of power transformer 16, it may be connected in the secondary winding side of the transformer if desired.

Also, although the bridge circuit is shown conductively coupled into the power line, it may be coupled into a power line by means of an additional transformer. In such a case, instead of connecting the bridge corners 44 and 48 directly into the power line as shown, the primary winding of the additional transformer is connected across these bridge corners with the secondary winding connected in series in the power line.

It is also contemplated to modify the form of the invention shown and described herein by employing a single winding for the two control windings 41 and 43. If this is done the single winding can have separate portions associated with two adjacent bridge reactors connected between the bridge output corners. In any event, the result will be to substantially reduce or short circuit the impedance across the bridge output under normal operating conditions, and to have relatively little effect on the operation of the bridge when the operating condition of the circuit varies from normal.

It is to be understood that the foregoing description and the accompanying drawings have been given only by Way of illustration and example, and that changes and alterations in the present disclosure, which will be readily apparent to one skilled in the art, are contemplated as Within the scope of the present invention which is limited only by the claims which follow.

What is claimed is:

1. In a voltage control system comprising a Wheatstone type bridge circuit containing two sets of opposed bridge impedances and two sets of opposed corners, means for impressing a voltage across one set of corners, and means for varying the magnitude of at least one set of said bridge impedances for producing a variable voltage across the other set of corners, said means being responsive to a selected electrical condition in the system which varies from a normal value to values above and below normal; the improvement which comprises other impedance varying means associated with two adjacent ones of said bridge impedances that are connected in series with each other between the said other set of said bridge corners, circuit means for deriving a signal responsive to the variations in said electrical condition, and means responsive to said signal and connected to said other impedance varying means for varying the magnitude of said two adjacent bridge impedances responsive to said selected electrical condition so that the magnitude of the said two adjacent bridge impedances is substantially at a minimum when said electrical condition is at its normal value.

2. In a voltage control system containing power input and output circuits; means for producing a correcting voltage, including a Wheatstone type bridge circuit containing two sets of opposed bridge impedances and two sets of opposed corners, and means for impressing a voltage across one set of corners, the other set of corners being connected in a circuit between the power input and output circuits; and first impedance varying means for controlling the balance conditions of said bridge circuit responsive to a selected electrical condition in the system, including a first and a second source of current for inversely varying the magnitudes of said sets of bridge impedances: the improvement which comprises second impedance varying means operatively associated with two adjacent ones of said bridge impedances that are connected in series with each other between said other set of corners, and means connected to said second impedance varying means for varyingthe magnitude of said two adjacent bridge impedances responsive to the sum of the currents flowing from said first and second sources to reduce the magnitude of each of said two adjacent bridge impedances to a minimum when said bridge circuit is substantially balanced.

3. In an electrical control system containing power input and output circuits; means for producing a correcting voltage, including a Wheatstone type bridge circuit swam having two sets of opposed bridge reactors and two sets of opposed corners, and means for impressing a voltage across one set of corners, the other set of corners being connected in a circuit between the power input and output circuits; and means for varying the magnitude of at least one set of reactors in response to variations in the magnitude of anelectrical condition to provide said correcting voltage across said other set of corners: the improvement which comprises control means including a control winding having portions thereof associated respectively with two adjacent ones of said bridge reactors that are connected in series circuit relationship between said other set of corners, means for producing a current directly proportional to said electrical condition above a pre-determined magnitude, means for producing a current inversely proportional to said electrical condition above said predetermined magnitude, said currents being substantially equal when said electrical condition is at another predetermined magnitude, and means responsive to said currents and connected to said control winding for reducing. the impedances of said adjacent reactors to substantially a minimum only when said currents are substantially equal. 1

4. In combination in a voltage control system, a power input circuit connectable to a supply voltage source; a power output circuit connectable to a load; meansfor producing a correcting voltage, including a Wheatstone type bridge circuit containing two sets of saturable core reactors and two sets of opposed corners, means for impressing a voltage across one set of corners, the other set of corners being connected in a circuit in series between the power input and output circuits; and means for varying the impedances of at least one of said sets of saturable core reactors responsive to a selected electrical condition in the system to produce said correcting voltage across said other set of corners, said last named means including a first source of D.C. current connected to one set of reactors and a second source of D.C. current connected to the other set of reactors; impedance control means asso ciated with two adjacent ones of said bridge reactors the adjacent ends of which are connected to one corner of said one set of corners and the opposite ends of which are connected respectively to said other set of corners, and means for varying the current through said control means in order to vary the impedances of said associated adjacent reactors responsive to the ,sum of the DC. currents flowing from said first and second sources of D.C. current to reduce the impedance of said adjacent reactors to relatively low values when said electrical condition is at a predetermined value, said current varying means including a magnetic amplifier containing two control windings one of which is connected to the first source of D.C. current and the other of which is connected to the second source of D.C. current.

5. In combination in a voltage control circuit, a power input circuit connectable to a supply voltage source; a power output circuit connectable to a load; means for producing a correcting voltage, including a Whcatstone type bridge circuit containing two sets of saturable core reactors and two sets of opposed corners, means for impressing a voltage across one set of corners, the other set of corners being connected in a circuit in series between the power input and output circuits; and means for varying the impedances of the saturable core reactors responsive to a selected electrical condition in the system to produce said correcting voltage across said other set of corners, said last named means including a first source of D.C. current connected to one set of reactors and a second source of D.C. current connected to the other set of reactors; an impedance control winding having portions thereof associated with adjacent ones of said bridge reactors that are connected serially between said other set of bridge corners; and means for energizing said impedance control winding to reduce the impedances of said adjacent bridge reactors to relatively low values when lid said electrical condition is at a pre-determined value including a self-saturating magnetic amplifier containing a power winding and two control windings, the control windings being wound so as to oppose the of the power winding; connections between the first source of D.C. current and one of said control windings; connections between the second source of D.C. current and the other of said control windings; and means connecting the output of the amplifier across the control winding associated with said adjacent bridge reactors so that the impedances of said adjacent bridge reactors varies inversely with the output of the amplifier.

6. In combination, two sets of opposed saturable core reactors, each set including two A.C. reactance windings and at least one D.C. control winding, the A.C. reactance windings of the sets being connected together to form a Wheatstone type bridge circuit having two sets of diameta rically opposite bridge corners, means for impressing an A.C. voltage across one set of bridge corners, means for controlling the balance conditions of said bridge circuit including said D.C. control windings to provide a variable voltage at the other set of bridge corners, control winding means in common with two adjacent ones of said AC. reactance windings which are connected in series with each other between said other set of bridge corners, means connected to said control winding means and responsive to the balance conditions of the bridge circuit for supplying current to said control winding means so as to reduce the impedance of said two adjacent reactance windings to a minimum only when said bridge circuit is in balance.

7. In an electrical control system containing means for producing a variable voltage, said means, comprising at least four bridge impedances connected together to form a Wheatstone type bridge circuit having two sets of opposed bridge corners, means for impressing a voltage across one set of bridge corners, and means for varying the value of at least one of said bridge impedances in response to an electrical condition of the system for producing said variable voltage across the other set of bridge corners; the combination with the voltage producing means of an impedance varying device having portions thereof associated with two adjacent ones of said bridge impedances serially connected between said other set of bridge corners for varying the values of saidtwo adjacent bridge impedances, means for producing a first signal directly proportional to changes in said electrical condition of the system above a predetermined magni-. tude, means for producing a second signal inversely pro: portional to said changes in said electrical condition above said predetermined magnitude, means for combining said first and second signals to produce a third signal, and means responsive to said third signal for energizing said impedance varying device to reduce the values of said two adjacent bridge impedances to a minimum when said electrical condition is at a predetermined value.

8. An electric control system comprising a power input circuit connectable to a supply voltage source, a power output circuit connectable to a load, means including a pair of conductors for connecting the power input circuit with the power output circuit, a bridge circuit including four impedances connected together to form two sets of opposed impedances and two sets of diametrically opposite bridge corners, means for impressing a voltage across one set of corners, means for varying at least one of said impedances in response to variations in the magnitude of an electrical condition of the system to provide a varible voltage across the other set of corners, means coupling said variable voltage in series with said supply voltage, and means for reducing said adjacent bridge impedances to relatively low values when said electrical condition is at a predetermined magnitude including impedance varying means associated with two adjacent ones of said bridge impedances that are serially connected between said other set of corners for varying the impedances thereof, first and second amplifiers, means for controlling the output of each of said amplifiers in response to variations in the magnitude of electrical condition of the system, said first amplifier producing an output which decreases when said electrical condition increases above another predetermined magnitude, said second amplifier producing an output which increases when said electrical condition increases above said other predetermined magnitude, a third amplifier having an output circuit connected to supply output current to said impedance varying means associated with said adjacent bridge impedances, and means for controlling the output current of said third amplifier in response to the outputs of said first and second amplifiers.

9. An electrical control system comprising a system power input circuit connectable to a supply source, a system power output circuit connectable to a load, a bridge circuit including four saturable core reactors connected together to form two sets of diametrically opposite reactors and two sets of diametrically opposite bridge corners, each set of reactors having at least one DC. control winding, means for impressing a voltage across one set of bridge corners, means including the other set of corners for connecting said bridge circuit in series between the power input and output circuits, a pair of impedance Varying windings associated respectively with adjacent ones of said saturable core reactors of the bridge circuit that are connected in series relationship between said other set of bridge corners, a pair of self-saturating magnetic amplifiers, each of said amplifiers comprising power inputand output circuits, a saturable core, a power winding on said core and connected between the amplifier power input and output circuits, and a control winding on said core, means for producing a signal responsive to an electrical condition of the system, means for supplying said signal to the control winding of each of said amplifiers for inversely affecting the outputs of said amplifiers, means for energizing one DC. control winding in response to the output of one of said amplifiers, means for energizing the other DC. control winding in response to the output of the other of said amplifiers, and means for energizing said impedance varying windings to reduce the impedances of said adjacent reactors to relatively low values when said electrical condition of the system is at a predetermined value including a third amplifier comprising power input and output circuits, a saturable core, a power winding on said core and connected between the power input and output circuits of said third amplifier, and a signal input circuit including control winding means on said core, means for energizing said signal input circuit of said third amplifier in response to the outputs of said pair of amplifiers, and means for energizing said pair of impedance varying windings in response to the output of said third amplifier.

10. An electric control system comprising a system power input circuit connectable to a supply source, a system power output circuit connectable to a load, a bridge circuit including four saturable core reactors connected together to form two sets of diametrically opposite reactors and two sets of diametrically opposite bridge corners, each set of reactors having at least one DC. control winding, means for impressing a voltage across one set of bridge corners, means including the other set of corners for connecting said bridge circuit in series between said power input and output circuits, a pair of impedance varying windings associated respectively with adjacent ones of said saturable core reactors of the bridge circuit that are connected in series relationship between said other set of bridge corners, means for controlling the balance conditions of the bridge circuit in response to the voltage across said system output circuit including a pair of self-saturating magnetic amplifiers, each of said amplifiers comprising power input and output circuits, a saturable'core, a power winding on said core and connected between the amplifier power input 16 and output circuits, a rectifier in series with the power winding for producing self-saturating M.M.F.s when a voltage is applied to the power input circuit, and a control winding on said core, means for producing a signal responsive to the voltage across said system output circuit, means for energizing the control winding of one of said amplifiers in response to said signal to produce desaturating M.M.F.s, means for energizing the control winding of the other amplifier in response to said signal to produce saturating M.M.F.s, means for energizing one DC. control winding in response to the output of one of said amplifiers, and means for energizing the other DC. control winding in response to the output of the other of said amplifiers, and means for reducing the impedances of said adjacent bridge reactors to relatively low values when said bridge circuit is in balance including a third amplifier comprising power input and output circuits, a saturable core, a power winding on said core and connected between the power input and output circuits of the third amplifier, a rectifier in series with the power winding for providing selfsaturating M.M.F.s when a voltage is applied to the power input circuit of the third amplifier, and a signal input circuit including at least two control windings on said core, means for energizing one of said latter control windings in response to the output of one of the amplifiers of said pair, means for energizing the other of said latter control windings in response to the output of the other amplifier of said pair, and means for energizing said pair of impedance varying windings in response to the output of said third amplifier.

11. In an electrical control apparatus including a four arm bridge circuit having two pairs of opposed bridge corners, reactor means in each arm, of the bridge circuit, each of said reactor means including a reactance winding connected between a corner of one of said pairs of opposed bridge corners and a corner of the other of said pairs of opposed bridge corners, means for connecting a voltage source to said one pair of op posed bridge corners, and means for controlling the balance conditions of the bridge circuit to provide a variable voltage at said other pair of bridge corners including a control winding on at least one of said reactor means, and means for supplying control current to said control winding, the combination therewith of control winding means on two of said reactor means that are disposed in adjacent arms of the bridge circuit, said reactance windings of said two reactor means being connected in series with each other between said other pair of bridge corners, said control winding means being adapted to vary the reactance values of said reactance windings of said two reactor means in response to current flow in said control winding means, and circuit means responsive tothe balance conditions of the bridge circuit connected to said control winding means for supplying current thereto to reduce the reactance values of the last named reactance windings to a minimum when the bridge circuit is substantially balanced.

, 12. In an electrical control system including a four arm bridge circuit having a pair of opposed bridge input corners and a pair of opposed bridge output corners, reactor means in each of said arms, each of said reactor means including a reactance winding connected between one of said bridge input corners and one of said bridge output corners, and control winding means, means for impressing a voltage across the bridge input corners, and circuit means responsive to an electrical condition in the system for varying the reactance of each of said reactance windings when said electrical condition varies from a predetermined value to provide a variable voltage at the bridge output corners, said circuit means including means for supplying control current to each of said control winding means, the combination therewith of additional control winding means on two of said reactor means that are disposed in adjacent arms of the bridge circuit, said reactance windings of said two reactor means being connected in series with each other between said bridge output corners, said additional control winding means being adapted to vary the reactance values of said reactance windings of said two reactor means in response to current flow therethrough, and other circuit means connected to said additional control winding means and responsive to changes in said electrical condition for supplying current tosaid additional control Winding means to simultaneously reduce the reactance values of the last named reactance windings to a minimum only when said electrical condition is substantially at said predetermined value.

13. An electrical control system comprising a bridge circuit having four arms interconnected to provide a pair of opposed bridge input corners and a pair of opposed bridge output corners, four impedance devices respectively connected in the four arms of the bridge circuit, means for impressing a voltage across the bridge input corners, and bridge control means for selectively balancing and unbalancing the bridge circuit to provide a variable voltage across the bridge output corners including impedance varying means associated with at least one of said bridge impedance devices for varying the impedance thereof, and other control means for reducing the impedances of two of said bridge impedance devices that are connected in series circuit relationship between said bridge output corners to substantially a minimum when the bridge circuit is balanced, said other control means including other impedance varying means associated with said two bridge impedance devices for varying the impedance thereof.

References Cited by the Examiner UNITED STATES PATENTS 2,931,968 4/60 Hilker 323-89 LLOYD MCCOLLUM, Primary Examiner.

MILTON O. HIRSHFIELD, Examiner. 

1. IN A VOLTAGE CONTROL SYSTEM COMPRISING A WHEATSTONE TYPE BRIDGE CIRCUIT CONTAINING TWO SETS OF OPPOSED BRIDGE IMPEDANCES AND TWO SETS OF OPPOSED CORNERS, MEANS FOR IMPRESSING A VOLTAGE ACROSS ONE SET OF CORNERS, AND MEANS FOR VARYING THE MAGNITUDE OF AT LEAST ONE SET OF SAID BRIDGE IMPEDANCES FOR PRODUCING A VARIABLE VOLTAGE ACROSS THE OTHER SET OF CORNERS, SAID MEANS BEING RESPONSIVE TO A SELECTED ELECTRICAL CONDITION IN THE SYSTEM WHICH VARIES FROM A NORMAL VALUE TO VALUES ABOVE AND BELOW NORMAL; THE IMPROVEMENT WHICH COMPRISES OTHER IMPEDANCE VARYING MEANS ASSOCIATED WITH TWO ADJACENT ONES OF SAID BRIDGE IMPEDANCES THAT ARE CONNECTED IN SERIES WITH EACH OTHER BETWEEN THE SAID OTHER SET OF SAID BRIDGE CORNERS, CIRCUIT MEANS FOR DERIVING A SIGNAL RESPONSIVE TO THE VARIATIONS IN SAID ELECTRICAL CONDITION, AND MEANS RESPONSIBLE TO SAID SIGNAL AND CONNECTED TO SAID OTHER, IMPEDANCE VARYING MEANS FOR VARYING THE MAGNITUDE OF SAID TWO ADJACENT BRIDGE IMPEDANCES RESPONSIVE TO SAID SELECTED ELECTRICAL CONDITION SO THAT THE MAGNITUDE OF THE SAID TWO ADJACENT BRIDGE IMPEDANCES IS SUBSTANTIALLY AT A MINIMUM WHEN SAID ELECTRICAL CONDITION IS AT ITS NORMAL VALUE. 